The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Emissions control is an important factor in engine design and engine control. Oxides of nitrogen, NOx, are known by-products of combustion. NOx are created by nitrogen and oxygen molecules present in engine intake air disassociating in the high temperatures of combustion, and rates of NOx creation include known relationships to the combustion process, for example, with higher rates of NOx creation being associated with higher combustion temperatures and longer exposure of air molecules to the higher temperatures. Reduction of NOx created in the combustion process and management of NOx in an exhaust aftertreatment system are desirable.
NOx molecules, once created in the combustion chamber, can be converted back into nitrogen and oxygen molecules in exemplary devices known in the art within the broader category of aftertreatment devices. However, one having ordinary skill in the art will appreciate that aftertreatment devices are largely dependent upon operating conditions, such as device operating temperature driven by exhaust gas flow temperatures and engine air/fuel ratio. Additionally, aftertreatment devices include materials, such as catalyst beds, prone to damage or degradation as a result of use over time and exposure to high temperatures.
Modern engine control methods utilize diverse operating strategies to optimize combustion. Some operating strategies, optimizing combustion in terms of fuel efficiency, include lean, localized, or stratified combustion within the combustion chamber in order to reduce the fuel charge necessary to achieve the work output required of the cylinder and increase engine efficiency, for example, by operating in an unthrottled condition, reducing air intake pumping losses. While temperatures in the combustion chamber can get high enough in pockets of combustion to create significant quantities of NOx, the overall energy output of the combustion chamber, in particular, the heat energy expelled from the engine through the exhaust gas flow, can be greatly reduced relative to other combustion strategies. Such conditions can be challenging to exhaust aftertreatment strategies, since aftertreatment devices frequently require an elevated operating temperature, driven by the exhaust gas flow temperature, to operate adequately to treat NOx emissions.
Aftertreatment devices are known, for instance, utilizing chemical reactions to treat exhaust gas flow. One exemplary device includes a selective catalytic reduction device (SCR). Known uses of an SCR utilize ammonia derived from urea injection to treat NOx. Ammonia stored on a catalyst bed within the SCR reacts with NOx, preferably in a desired proportion of NO and NO2, and produces favorable reactions to treat the NOx. One exemplary embodiment includes a preferred one to one, NO to NO2 proportion, and is known as a fast SCR reaction. It is known to operate a diesel oxidation catalyst (DOC) upstream of the SCR in diesel applications to convert NO into NO2 for preferential treatment in the SCR. Continued improvement in exhaust aftertreatment requires accurate information regarding NOx emissions in the exhaust gas flow in order to achieve effective NOx reduction, such as dosing proper amount of urea based on monitored NOx emissions.
Urea injection used for the aftertreatment of an engine has certain difficulties. Factors such as urea temperature, pump pressure, and injector nozzle obstructions can modify the delivery of urea to the exhaust gas flow thereby preventing the delivery of the appropriate amount of urea. This results in either wasteful use of the stored urea, by providing an excess beyond the required amount, or too little to effectively treat the amount of exhaust produced.